Angiogenesis is a highly regulated process occurring due to reactions to various angiogenic factors, such as hypoxia and low pH, as well as growth factors, cytokines, and other physiological molecules (Folkman and Shing, J. Biol. Chem., 267, 10931, 1992). To develop new blood vessels, the angiogenesis mechanism requires cooperation with various molecules that regulate decomposition and reconstruction, migration, proliferation, differentiation, and tube-formation of extra-the cellular matrix (ECM). In addition, after the initiation of angiogenesis, angiogenesis promotion factors including VEGF, bFGF, PDGF, etc. activate by stimulating receptors on cell surfaces. The activated cells induce cellular proliferation, the expression of cell adhesion molecules, the secretion of proteolytic enzymes, and cellular migration and infiltration. In addition, various molecules including integrins for cell adhesion, selectin, immunoglobulin gene superfamily members, and enzymes for protein decomposition, such as matrix metaloprotease for ECM decomposition and serine protease promote the proliferation and infiltration of cells (Brooks, Eur. J. Cancer, 32A, 2423, 1996), Further, lumen formation and differentiation to mature blood vessels are induced by a signal transduction mechanism mediated by the cell surface receptor that can interact with ECM components and lytic factors, resulting in angiogenesis.
Recently, attempts have been actively made to treat angiogenesis-mediated diseases such as cancer, rheumatoid arthritis, psoriasis, ulcer, ischemia, atherosclerosis, myocardial infarction, angina pectoris, and cerebrovascular disease, using a factor that induces or inhibits angiogenesis (Folkman J., J. Nat. Med., 1:27, 1995; Jackson J. R., et al., FASEB J., 11:457, 1997; Risau W., Nature, 386:671, 1997; Bussolino, F., et al., Trends Biochem. Sci., 22:251, 1997; Hanahan D., et al., Cell, 86:353, 1996).
In normal adults, endothelial cells composing blood vessels are replaced every 47 to 20,000 days, which is strictly regulated. In general, angiogenesis inhibitors, such as thrombospondin-1, platelet factor-4, and angiostatin, and the angiogenesis promoters, such as vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF), are quantitatively balanced, whereby angiogenesis is not induced. However, when a wound or cancer is developed, the balance between angiogenesis inhibitors and angiogenesis promoters is broken in order to induce regeneration of wounded tissue and growth of cancer and new blood vessels are formed. In this case, over-expression of angiogenesis promoters is induced.
Accordingly, excessive blood vessel formation can be a major cause of disease exacerbation and non-formation of the blood vessels can also be a cause of severe diseases. Angiogenesis is considered to be an essential phenomenon for wound healing or tissue regeneration. For example, a lack of angiogenesis in the placenta can be a major cause of miscarriages. In addition, necrosis, ulcer, and ischemia due to a lack of angiogenesis may cause dysfunction in tissues or organs or even death.
Therefore, it is very important to induce or promote formation of new vessels so as to reduce tissue damage caused by a hypoxic or mal-nutrition state due to non-formation of blood vessels and to induce smooth tissue regeneration.
In particular, a wound repair process essential for regeneration of wounded skin tissue should be necessarily accompanied by angiogenesis. In the early stage of wound repair, inflammation occurs due to cellular necrosis and blood vessel destruction. After such inflammation, a series of processes such as formation of biological mediators, such as kallikrein, thrombin, and plasmin, along with the escape of blood components, platelet activation, blood coagulation followed.
Ischemia is a partial blood shortage symptom wherein normal angiogenesis is not sufficiently performed and thus blood supply is stopped, whereby cell damage is caused. Ischemic diseases is a generic term including all diseases induced by such ischemia. For example, examples of ischemic diseases include lower limb ischemia, ischemic stroke, ischemic colitis and cardiovascular diseases. In the case of ischemic stroke or ischemic heart disease, as a representative ischemic diseases, brain cells and cardiocytes are damaged by ischemia wherein cerebral arteries or coronary arteries are obstructed by a thrombus or arteriosclerosis and a blood flow rate is decreased to a threshold or lower, and finally cellular necrosis occurs, whereby cerebral infarction and myocardial infarction occur. A blood supply shortage due to ischemia also causes various ischemic diseases such as ischemic heart failure, ischemic enteritis, ocular disease, and lower limb ischemia.
As methods of treating such ischemic diseases, there are a pharmacotherapy method and a coronary angioplasty method of distending narrow blood vessels by means of a stent. In the case of the heart, artery bypass surgery may be conducted. However, such treatments are not suitable methods when blood vessels are extremely hardened, all blood vessels available for transplantation were used, or relapsing continues despite repeated arterioplasty by restenosis. Therefore, to overcome the limitations of such operative treatments, research on a method of treating ischemic diseases characterized by directly injecting vascular endothelial cells into tissue and thus allowing new blood vessels to be formed in a damaged region is underway.
Angiogenesis with vascular endothelial cells has great clinical significance with regard to ischemic blood vessel diseases. However, when vascular endothelial cells derived from different species are injected, problems related with biocompatibility may occur. To address such problems, attempts to derive autologous vascular endothelial cells from embryonic stem cells or induced pluripotent stem cells are being made. However, a defined differentiation method does not yet exist and limitations due to technically difficult embryoid body manipulation, low differentiation efficiency, contamination possibility by feeder cells, etc. are present. To overcome such limitations, a method of directly transdifferentiating differentiated cells into a different cell type is actively underway. With regard to this, it was reported that it was possible to transdifferentiate beta cells of the pancreas, nerve cells, cardiac myocytes, hepatocytes, etc. In 2012, Ginsberg et al. showed that, by introducing ER71, FLI1, and ERG1 into human amnion cells and inhibiting TGFβ, the human amnion cells were transdifferentiated into endothelial cells.
However, Ginsberg et al. failed to transdifferentiate adult cells into endothelial cells. They only accomplished transdifferentiation of amniotic cells. Their study has limited clinical relevance because amniotic cells are not readily available and because this approach still possesses the critical limitations of embryonic stem cells-derived ECs such as immunogenicity and possible allograft rejection. Moreover, amniotic cells isolated from amniotic fluid of midgestation human fetuses should be distinct from fully differentiated mature cells. Thus, the clinical application of their study is quite limited.